This is a Nailkona Stage Entry under 37 C.F.R. xc2xa7 371 of PCT/EP00/04503, with an international filing date of May 18, 2000, the complete disclosure of which is incorporated herein by reference.
The present invention relates to a unit for actuating the rod of a closure element in a mould for the injection-moulding of plastics material.
An injection-moulding mould provided with a conventional actuating unit is shown in partial section in FIG. 1. A heated nozzle 10 is mounted beneath a heated plate 11 in order to receive the molten plastics material from a supply duct 12 and to inject it into a moulding cavity 13 through an injection duct 14 and a lower injection hole 15.
A closure rod 16 slides coaxially inside the injection duct 14, the axial position of the rod 16 being controlled by an upper actuating unit, generally indicated 17 and housed in a cavity 18 formed in an upper plate 19. The actuating unit 17, which is pneumatically operated, comprises a cylinder 20 in which a piston 21, to which the upper end 16a of the rod 16 is connected, can slide vertically.
The vertical movement of the piston 21 and of the rod 16 is brought about by sending an operating fluid (air) alternately through an inlet duct 22 and an inlet duct 23 which are formed in the upper plate 19 and communicate with the cavity 18 in upper and lower positions, respectively, in order to admit the operating fluid to the cylinder 20 through a duct 24 or a duct 25, so as to raise or lower the piston 21. As is known, in the lowered position (not shown), the lower end 16b of the rod 16 blocks the injection hole 15 whereas, in the raised position (as shown in FIG. 1), the injection hole is open to allow plastics material which is injected from the duct 14 of the nozzle 10 to pass through to the moulding cavity 13.
To prevent the injected material From reaching the actuating unit 17, a tempered steel sealing bush 26 is provided and is housed in a cavity 27 in the hot plate 11; the sealing bush 26 is fitted on the slidable rod 16. A reaction ring 28 is interposed between the hot plate 11 and the upper plate 19. A closure cover 29 is fixed to the upper plate 19 to restrain the actuating unit 17 at the top.
A conventional actuating unit of this type requires numerous seals; in addition to two seals 30 and 31 between the piston 21 and the cylinder 20, there is a seal 32 between the cover 29 and the upper plate 19, there are two seals 33 and 34 between the upper plate 19 and the stationary cylinder 20, and there is a further seal 36 mounted on an enlarged element 37 acting as a stop for the rod 16.
Injection moulds provided with actuating units configured as described above have a disadvantage of a practical nature when it is necessary to perform adjustment or maintenance operations on one or more closure rods, for example, when it is necessary to replace a worn rod or to clear an injection duct of accumulations of waste. For example, in order to gain access to an injection nozzle, it is necessary to remove the upper plate 19 and with it all of the closure units of the various injection nozzles of which there may be a considerable number (48 or more) in cases in which the mould comprises several nozzles and numerous moulding cavities. During the removal of the upper plate 19, all of the rods of the various closure units are thus removed.
Since an operation of this type is performed hot to enable the rods to be removed, the plastics material tends to rise through the sealing bushes 26 and thus to be deposited on surfaces which should, however, remain clean. Once the maintenance or adjustment operation has been performed on the nozzle or nozzles which required it, it is therefore necessary to clean the surfaces on which plastics material has been deposited during the operation, for all of the nozzles of the mould, before the upper plate with the respective rods and closure units is reassembled.
As will be understood, the removal of the upper plate, complete with closure units and rods, is a complex operation but, above all, careful cleaning at many points of the mould as referred to above requires long periods of inactivity of the apparatus and is therefore expensive.
Moreover, during the reassembly of the closure units, it is necessary to pay particular attention to the various seals, ensuring that they are positioned correctly before closing the mould again.
An object of the invention is to provide an actuating unit in which the piston can be released from its rod easily without requiring the latter to be removed from the nozzle.
An object of the present invention is to propose an actuating unit which is easy to install and to maintain, and which can prevent the problems of the prior art discussed above.
A further object of the present invention is to provide an actuating unit having an improved and simplified system for adjusting the travel of the closure rods; in particular, the adjustment of the travel and the removal of the rod of a particular actuating unit should be independent operations.
This further object is achieved, again according to the invention, by an actuating unit having the characteristics defined in claim 4.
Yet another object of the invention is to mould products which do not bear obvious marks on their surfaces in the regions of the injection points when these points lie on surfaces which are not perpendicular to the closure rod.